Method of producing a cemented carbon electrode joint



METHOD OF PRODUCING A CEMENTED CARBON ELECTRODE JOINT 3 Sheets-Sheet 5 Filed April 27. 1960 Fig.5

United States Patent 3,140,967 METHOD OF PRODUCING A QEMENTED CARBON ELECTRQDE JOINT Waldemar Kaufmann, Erich Fitzer, Hans-Joachim Pileiderer, Alfred Pelz, and Wilfried Hub, all of Meitingen, near Augsburg, Germany, assignors to Siemens-Planiawerke Aktiengesellschaft fiir Kohlefabrikate, Meitingen, near Augsburg, Germany, a German corporation Filed Apr. 27, 1960, Ser. No. 24,965 Claims priority, application Germany Apr. 30, 1959 6 Claims. (Cl. 156-91) Our invention relates to coaxial junctions of carbon electrodes for electric furnaces and other electric purposes, and particularly to electrode joints that comprise a double-conical threaded nipple plug of electrode material whose two conically tapering portions are screwed together with respective conical socket recesses in the front faces of the two coaxially aligned electrodes of graphite or other carbon material. Such joints serve to connect a new electrode with the end of a nearly consumed electrode to permit continuous furnace operation by replenishing the electrode material in accordance with the rate of consumption. It is known to provide such nipple-screw junctions with a putty-like cement for improving the reliability of the electrode connection.

It is an object of our invention to facilitate and improve the cementing of electrode junctions of the type mentioned, without requiring the provision of additional storage spaces and storage devices for accommodating and properly applying the cement.

To this end, and in accordance with our invention, a quantity of cement is wrapped in a bag or other envelope, and the enveloped cement is then placed into the space between the socket bottom of an electrode and the screw nipple before these two parts are joined together. The envelope of the cement consists of a material which is sufiiciently plastic or flaccid to permit deformation of the cement mass without destruction of the envelope while the envelope is being handled prior to its use in the joint. The envelope material consists of substance which can readily be destroyed thermally and/ or chemically at the proper stage of use. Preferably, the envelope material is thermally and mechanically resistant at room temperature, especially at temperatures below 60 C., but becomes thermally decomposed at elevated temperatures below 100 C. Particularly suitable is flaccid synthetic foil material, such as polyethylene foil. However, the envelope may also be formed by coating the mass of cement with a hardened layer of gelatin. Other wrappers or envelopes satisfying the above-mentioned conditions are likewise suitable. They are preferably given the shape of flat bags or cylindrical containers.

The cement to be used for the purposes of the invention may be any of the known and commercially available pasty or putty-like carbon pastes and carbon cements such as those listed in Table 51, page 376 of Mantell, Industrial Carbon, 1946; D. Van Nostrand Company, Inc., New York. However, it is preferable to modify the basic cement mass by admixing catalyzers and/ or swelling agents as described further below.

According to other features of our invention, the components of the electrode junction are especially designed for the above-mentioned cementing method. More specifically, one or more grooves are provided which extend transverse to the turns of the screw threads of nipple and socket from the front face of the nipple plug or from the socket bottoms of the electrodes and which may also extend over the front face of the nipple plug. The grooves in the nipple and/ or in the threaded walls of the electrode sockets permit the cement to enter into all turns of the screw thread. The grooves also constitute a pressure relief valve which reduces excessive gas pressure as may result from the condensation and carbonization of the cement or from the swelling agents that may be contained in the cement. Thus the grooves prevent bursting of the electrode joints, particularly of the electrode socket walls. As a rule, it suffices to provide the nipple plug with one distributor groove. However, it is preferable to symmetrically distribute two or more such grooves over the nipple periphery.

According to another feature of the invention, the grooves in the nipple and/ or in the electrode sockets extend up to the equator, i.e. the largest diameter of the double-conical plug or to the butt faces of the electrodes. This permits the cement, when swelling to penetrate into the gap between the adjacent butt faces of the electrodes, thus providing for additional cementing action. Such cementing between the electrode butt faces is preferably promoted by providing the butt faces with grooves or other recesses which preferably extend in radial directions and communicate with the socket space.

The foregoing and other objects, advantages and features of our invention, said features being set forth with particularity in the claims annexed hereto, will be apparent from, and will be mentioned in the following description of the embodiments of the invention illustrated by way of example on the accompanying drawings in which:

FIG. 1 is a longitudinal section in an axial plane of an electrode nipple junction, only the adjacent end por tions of the two electrodes being shown.

FIG. 2 is a section along the line IIII in FIG. 1.

FIGS. 3a, 3b and 3c are explanatory and serve to illustrate different stages of the method according to the invention.

FIG. 4 shows one elctrode joined with a nipple plug during an intermediate stage of the joining operation.

FIG. 5 is a schematic perspective view of a cemented electrode joint, partially cut away.

The same reference numerals are used in all illustrations for denoting corresponding elements respectively.

Referring to FIG. 1, the two electrodes 1 and 2 joined with each other are provided with inten'orly threaded sockets 3, 4 extending inward from the respective butt faces. The sockets are in threaded engagement with a double-conical nipple plug 4 consisting preferably of the same graphite or other carbon material as the electrodes. The joint further contains cement at 9 and 10.

When the junction is being made, a quantity of cement within an envelope is placed between the front faces 5, 6 of the nipple plug 4 and the respective bottoms 7 and 8 of the electrode sockets (FIGS. 3a, 3b, 3c). After the filled envelopes 9, 10 are placed into the socket spaces, the electrodes and the nipple plug are tightly screwed together. As Will be explained, it is preferable to keep the enveloped cement quantity somewhat smaller than the volume of the socket space remaining when the electrodes are fully screwed together, so that the envelope remains intact and becomes destroyed thereafter by thermal and/or chemical action.

The nipple 4 is provided with four longitudinal grooves 12 (FIGS. 1, 2) whose depth is equal to that of the screw thread.

The front faces of the electrodes 1 and 2 are provided with radial grooves or recesses 13, 14 (FIG. 1).

FIGS. 3a, 3b, 3c illustrate how the tautly filled cement envelope permits symmetrizing the nipple plug relative to the electrode butt faces, as will be further explained below.

FIG. 4 shows how a tautly filled cement bag is pressed fiat by screwing the nipple plug together with an electrode and thus can be ruptured for releasing the cement simply by mechanical action.

FIG. illustrates a completed junction after the junction has been heated and carbonized. It can be seen in the cut-out of electrode 2 how the cement is distributed from the socket bottom through the distributor groove 12 into the nipple threads and into the gap between the butt faces of the electrode.

Since the envelope, when handling the cement prior to its use in the electrode junction, is not subjected to appreciable stresses it can be given relatively small thickness so that it will burst open simply due to the mechanical pressure occurring when the nipple junction is screwed tight. As mentioned, however, it is preferable to use an envelope material which becomes destroyed by thermal and/ or chemical effect after the nipple junction is tightly screwed together and is heated when the electrode is inserted into the furnace. The use of an elastic or flaccid envelope material has the advantage that the plasticity of the cement need not meet exacting requirements because the mass of cement is at first kept together by the envelope.

It is preferable to use a cement mass which contains a pitch component, a component of synthetic plastic which thermally hardens prior to carbonization of the pitch, and if desired also a component of granular solid material, and additionally a component consisting of a swelling agent. When the cement-filled envelope is placed into the joint and the joint is tightly screwed together, the subsequent heating of the joint causes destruction of the envelope, the cement swells under the effect of the swelling component, and an early, preliminary hardening effect takes place due to setting of the synthetic plastic component.

During further heating, the pitch and the synthetic substance become carbonized thus securing a permanent high cementing strength of the joint. The carbon surfaces to be cemented are preferably pre-impregnated with a catalyzer to promote hardening, so that a preliminary hardening occurs rapidly as soon as the envelope is destroyed. The hardening catalyzer has a decomposing effect upon the envelope and thus aids in the rupturing action mainly caused by heating or mechanical destruction.

We found that benzol sulfo-acid hydrazide is particularly suitable as a swelling agent in an amount of 2 to by weight relative to the total weight of mixed cement mass. Also suitable as a swelling agent is ammonium phosphate in an amount of 2 to 10%.

Preferably used as catalyzer to promote hardening is chloranil which acts as an oxidizer upon polyethylene envelope material and thus destroys the initially insulating intermediate polyethylene layer between the carbon and the cement mass. The packaged quantity of cement is preferably so rated that the interspace between the front face of the nipple plug and the bottom of the electrode socket is filled only up to about four-fifths.

Applicable in lieu of the cement substances so far mentioned is also a pure synthetic-resin graphite cement. The viscosity of the cement being used is practically of no importance, no particular requirements being placed upon the consistency of the cement. Thus, for example, a heterogeneous cement composed of synthetic-resin, pitch and fine-granular solids may be used. We found it particularly advantageous to employ a cement that contains a thermally hardenable synthetic-resin capable of having a swelling efiect upon the finely ground pitch. Such a cement is further described below.

Such resin-pitch cements are preferably combined with swelling agents as already mentioned. The swelling agent can be admixed in form of inorganic or organic compounds. However, when swelling agents are used, the plastic cement mass should have such a surface tension that foaming of the cement takes place so that the swelling agent does not escape in form of large bubbles without actually swelling the cement. Particularly suitable as cements with swelling agents are the above-mentioned resin-pitch combinations particularly when the pitch is present in preliminarily somewhat swelled condition. We have further found it to be particularly advantageous if the cement contains hardening catalyzers which augment the thermal hardening of the cement by superimposed chemical reaction (condensation) after the cement has become distributed throughout the joint. According to another feature of the invention, we use swelling agents which, when becoming decomposed, evolve a hardening catalyzer. This ensures that hardening of the cement occurs only after the foaming or swelling action.

Such combined-substance cements, when heated, develop into a foam-type coke. Such a foamy coke rigidly reinforces the junction because the coke during heating, breaks up into splinter-shaped particles which block the screw threads and thus reliably prevent loosening of the threaded junction even under vibratory stresses. On the other hand, the foamy coke permits loosening the screw thread by brute force because the rotary unscrewing motion then grinds the coke into much finer particles. This is a fundamental improvement over the locking cements conventionally used which often result in breaking the nipple plug or electrode socket when an attempt is made to forcibly open the joint. The desirability of unscrewing an electrode nipple joint often occurs in steel mills in the event an arc-furnace electrode assembly is damaged. If a damaged assembly, involving for example a broken nipple plug, can be unscrewed, the electrode can readily be used again. This is made possible when using a cement combination in accordance with the present invention. In contrast thereto, a permanently rigid blocking of the junction by other cements requires severing of the electrode which must then be provided with a new screw socket.

As mentioned, the packaged cement is inserted prior to screwing the nipple plug and electrodes together. The electrode socket and the nipple plug are preferably so dimensioned that the cement envelope can be placed into the interspace between the socket bottom and the nipple front. It is preferable to keep this interspace so large that, when the joint is fully screwed together, the envelope is flattened but still intact, and is thereafter destroyed only by breaking and by the conjoint effect of the swelling agent contained in the cement. The heating temperatures occurring in practice are in the vicinity of C. At these temperatures the cement, after being released from the envelope, penetrates into the completed nipple-screw junction and hence cements only the gap spaces that remain between nipple and socket threads after the junction is tightly screwed together. This has the further advantage that the respective interspaces between the nipple plug and the socket bottoms need not have the exactly accurate dimensions. The above-described grooves take care that the viscous mixture of cement and swelling agent is well distributed, with the result of forming not simply a cement cake between nipple front face and socket bottom, but spreading the cement between the mutually engaging turns of the severed thread between plug and socket, leaving only a foamy coke mass between nipple front and socket bottom.

In contrast thereto, the known cementing methods require giving the cement a very accurate dosage. Furthermore, the hollow spaces between the nipple front faces and the socket bottoms must be very accurately machined, which aggravates the production requirements and increases the cost.

The cementing method according to the invention also achieves the following advantage. The nipple plug in electrode joints must be seated symmetrically between the two electrodes to prevent damage if the thermal expansion of the nipple plug differs from that of the socket-adjacent electrode portions. When the nipple plug is screwed asymmetrically into the sockets, i.e. when the nipple plug is first screwed into the lower socket until both flanks of the screw thread on the nipple tightly engage those of the socket, there is the danger that the nipple may be too tight in the lower electrode socket so as to cause bursting of the socket, whereas the nipple is too loose in the upper socket and hence may drop out of that socket when extreme stresses are encountered. Various auxiliary devices have been proposed to prevent such an asymmetrical screw connection. However, the same effect can be achieved by virtue of the cementing method according to the invention without requiring any auxiliaries. This will be explained presently.

When using cement-filled envelopes or bags of the size described above, the nipple, when being screwed by hand into the first socket, cannot be screwed down to such a depth that the nipple thread will abut on both flank sides against the socket thread. That is, when the nipple is screwed in by hand, the cement bag remains intact and serves as a stop so that the equator (laregst diameter) of the double-conical nipple plug stays away from the butt face of the electrode (FIG. 3a). That is, the nipple is cushioned on the cement bag. Now the second cement bag (9 in FIG. 3b) is placed upon the free nipple front face and the second electrode socket is screwed on to the nipple, likewise without exerting major force, until the socket bottom abuts against the cushion formed by the second cement bag. Now, the joint must be tightened and this can be done only by bursting the two cement bags (FIG. 30). This takes place in both sockets at approximately the same time. The further screwing-together of the joint has the effect of pressing the cement out of the bag into the hollow spaces between nipple plug and socket. During subsequent heating, the cement is subjected to swelling action and passes through the grooves 12 into any clearance remaining between the turns of the interengaging screw threads.

According to another mode of applying the invention, the bag or other package of cement is so tautly filled that its height (thickness) of the envelope, even when subjected to light screw pressure, is larger than the largest clearance between the socket bottom and the nipple front face when the nipple and electrode are screwed together. Then, the further tightening of the screw joint is possible only by mechanical destruction of the plastic cement bag (FIG. 4).

It has been found in practice that an electrode joint made according to the invention, and hence having the cement distributed throughout the screw threads, solidifies very rapidly to the desired high strength and then reliably preserves its high cementing strength during operation of the electrodes. When furnace electrodes, in cold condition, are being inserted into an arc furnace the first phase of furnace operation during which the metal in the furnace is being melted, imposes an extremely high current load upon the electrodes. During this initial period, the electrode joints are subjected to particularly great vibratory stresses. A large portion of the electric current passes from the electrode socket through the nipple threads and the nipple body. The high contact resistance due to the reduced conducting cross section in the screw threads produces an appreciable amount of Joules heat. This initially great amount of heat is particularly favorable for the desired rapid hardening of the cement located in the screw threads. According to a further feature of the invention this effect can be augmented by departing from the conventional concepts in providing the butt faces of the electrodes with an insulating layer, for example an inserted thin foil of paper or synthetic plastic. Such an insulating gasket of carbonaceous material has the effect that during the first few minutes of furnace operation the entire current will pass through the nipple plug thus causing a very rapid heating of the cement. Such rapid heating also has the effect of rapidly carbonizing the insulating foil between the electrode faces which thus become conductive, whereafter the current also passes directly between the adjacent faces of the electrodes.

In the following three examples of cement-compositions are cited in order to illustrate what kind of cements will be applicable to carry out our above described inven tion. Although appearing extremely different as to their composition, all of these three cements disclosed below have proved to be outstandingly suitable for the said purpose.

It must be emphasized that the components have to be mixed very thoroughly at room temperature (not exceeding 25 C.) lest an undesirable hardenmg reaction occurs. The shelf life of the ready-to-use cements 1s fairly good, In some cases even outstandmgly good. So has the cement made according to Example No. 1 a shelf life of 6-8 months at room-temperature.

Example 1 Parts Description of Component per component Remarks weight (a) Carbonaceous 5-15 Coke (preferably Dusty wastes from solid. petrol coke) powcoke milling equipdered to a grain ment are generally size Resisuitable. tli ge on ignition:

(b) Pitch 40-65 Hard coal tar pitch:

softening point (according to Kraemer-Sarnow) 160175 (3., quinoline-insolubles, 20-30% powdered to a grain size 0.2 mm. (0) Synthetic 30-45 A resin hardenable resin. by acids: viscosity 2001000 cp. at 20 0., preferably a iufur-acroleineindene-condensate according to German Patent (DAS) 1,048,413. ((1) Swelling 2-6 Hydrazide of henagent. zene-sulfonic acid. (d) Hardening 0 Upon decomposiagent. tion of the swelling agent (between 70- 0.) products are formed, which act as hardening agents.

Example 2 Parts Description of Component per component Remarks weight (a) Carbonaceous 10-25 Graphite (prefer- Dusty wastes from solid. ably artificial the machining of graphite) powgraphite electrodes dered to a grain are generally size 60 Resisuitable. due on ignition I 0.8%. (b) Pitch 20 Soft coal tar pitch:

softening point (according to Kraemer-Sarnow), 50-60" 0., powdered to a grain size 0.5 mm. (c) Synthetic 50-65 Phenol-formalderesin. hydc-condensate (Resol): viscosity, 10001500 cp. at 20 0. Hardenable by acids. 3 Ammonium-phos- (d) Swelling phate.

agent. 3 Ammonium-carbonate. (e) Hardening Finely dispersed This hardening agent. suspension of 20 agent must not be parts p.w. of added to the cechloranile in 80 ment itself but has parts p.w. of an to be brushed to aqueous solution the surfaces to be (1-2%) of ruethyljoined afterwards. cellulose. If possible allow this hardening paint to dry be fore applying the cement.

1 As required.

Example 3 Parts Description of Component per component Remarks weight Silieides of titanium or iron ('liSi or (a) Solids FeSi) grain size 5 Natural graphite: I grain size 100 1. (b) Pitch 40-50 Hard coal tar pitch -20% quinoli11einsoiubles softening point (according to Kraemer-Sarnow) 120- 150 0., grain size 0.2 mm. (c) Resmlike -60 Furturyl alcoho1 plastieizer. (d) Swelling 4-8 Graphite oxyde agent. (finely powdered). (e) Hardening.-. 0.5-2 Aqueous solution of agent. phosphoric acid containing 10% aPO4.

We claim:

1. The method of symmetrically and coaxially releasably joining two carbon electrodes, comprising (1) forming a double-conical threaded nipple plug having an equator portion midway between its ends,

(2) proportioning the distance from the equator portion to each of said nipple plug ends a predetermined selected distance,

(3) forming in the opposing faces of said electrodes threaded female conical socket portions each having a predetermined depth greater than said predetermined distance of said nipple plug,

(4) forming the diameter of said equator portion substantially equal to the greatest diameter of said socket portions,

(5) providing a thermally hardenable cement of selected chemical properties and containing a carbonizable substance,

(6) encasing a measured dosage of said cement in a flaccid and readily deformable plastic envelope, and

also inserting in said envelope a normally inactive swelling agent activatable by heat and being thermally decomposable to evolve a hardening catalyst for said cement.

(7) obtaining a symmetrical positioning of the longitudinal halves of said nipple plug in said electrode sockets by forming said envelope with a filled thickness greater than the difference between said socket depth and said nipple plug predetermined distance,

(8) placing one of said envelopes in one of said sockets in one of said electrodes,

(9) tightening said nipple plug into said socket portion against said first envelope until the latter is slightly deformed but not ruptured,

(10) placing another envelope of said cement and swelling agent on the open end of said nipple plug,

(11) screwing a second electrode with its socket into threaded engagement with said nipple plug until said second envelope is slightly deformed but not ruptured,

(12) subjecting said joints to envelope-destroying conditions to release said cement into said joints by screwing said electrodes on said nipple plug until said equator portion substantially abuts the periphery of the greatest diameter portion of each of said socket portions, and

(13) subjecting said joint to suflicient heat to activate the swelling agent, causing the cement to fill the threads and harden.

2. The method of claim 1, said heat being generated by passing an electric current through said joined electrodes in such manner that the greater portion of the current fiow goes first through said nipple plug and then through the adjoining surfaces thereof.

3. The method of claim 2, wherein the current control is achieved by placing an insulating but heat-carbonizablc substance between the adjacent faces of the electrodes to thereby cause the electric current to flow first through said nipple plug, the heat generated thereby causing said substance to carbonize so as to provide substantially uniform current throughout all areas of the joint.

4. In the method according to claim 2, said swelling agent consisting of benzol sulfo-acid hydrazide.

5. In the method according to claim 4, said swelling agent forming 2 to 10% of the total weight of the enveloped cement mass.

6. The method according to claim 4, said step of subjecting said joints to envelope-destroying conditions comprising tightening the two electrodes onto the nipple plug threads and against the nipple plug ends to thereby destroy said two envelopes not later than when placing said joined electrodes into current-conducting operation, so that the individual cement masses are releases into said joints.

References Cited in the file of this patent UNITED STATES PATENTS 1,743,888 Hamister Jan. 14, 1930 2,093,390 Wyckoff Sept. 14, 1937 2,421,105 Warren May 27, 1947 2,510,230 Johnson et al June 6, 1950 2,684,318 Meek July 20, 1954 2,805,879 Thomas Sept. 10, 1957 2,829,502 Dempsey Apr. 8, 1958 2,836,806 Stroup May 27, 1958 2,894,776 Johnson July 14, 1959 2,952,129 Dempsey Sept. 13, 1960 3,048,433 Doetsch Aug. 7, 1962 FOREIGN PATENTS 739,013 Great Britain Oct. 25, 1955 

1. THE METHOD OF SYMMETRICALLY AND COAXIALLY RELEASABLY JOINING TWO CARBON ELECTRODES COMPRISING (1) FORMING A DOUBLE-CONICAL THREADED NIPPLE PLUG HAV ING AN EQUATOR PORTION MIDWAY BETWEEN ITS ENDS. (2) PROPORTIONING THE DISTANCE FROM THE EQUATOR PORTION TO EACH OF SAID NIPPLE PLUG ENDS A PREDETERMINED SELECTED DISTANCE, (3) FORMING IN THE OPPOSING FACES OF SAID ELECTRODES THREADED FEMALE CONICAL SOCKET PORTIONS EACH HAVING A PREDETERMINED DEPTH GREATER THAN SAID PREDETERMINED DISTANCE OF SAID NIPPLE PLUG,, (4) FORMING THE DIAMETER OF SAID EQUATOR PORTION SUBSTANTIALLY EQUAL TO THE GREATEST DIAMETER OF SAID SOCKET PORTIONS, (5) PROVIDING A THERMALLY HARDENABLE CEMENT OF SELECTED CHEMICAL PROPERTIES AND CONTAINING A CARBONIZABLE SUBSTANCE, (6) ENCASING A MEASURED DOSAGE OF SAID CEMENT IN A FLACCID AND READILY DEFORMABLE PLASTIC ENVELOPE, AND ALSO INSERTING IN SAID ENVELOPE A NORMALLY INACTIVE SWELLING AGENT ACTIVATABLE BY HEAT AND BEING THERMALLY DECOMPOSABLE TO EVOLVE A HARDENING CATALYST FOR SAID CEMENT. 